SPECTROSCOPY AND NONRADIATIVE PROCESSES IN GD3+, EU3+ AND TB3+ TROPOLONATES

Complexes of Eu3+, Gd3+, and Tb3+ with tropolone (tropolone = 2-hydrox y-2,4,6-cycloheptatrien-1-one) were synthesized and characterized by C and H elemental analysis, infrared vibrational spectroscopy, X-ray po wder diffraction analysis and Raman vibrational spectroscopy. The resu lts show that the complexes are isostructural and have the formula [Ln (trop)(3)], where Ln = Eu3+, Gd3+ or Tb3+ and trop = tropolone. The ch aracteristic luminescence of the Eu3+ and Tb3+ ions was not observed e ither upon ligand excitation (300-380nm) or upon direct f-f excitation , not even at 4.2 K. In order to understand this phenomenon the spectr oscopic properties of the Gd(trop)(3) complex (absorption spectra at 2 98 K, excitation and emission spectra from 4.2 to 298 K and decay time measurements from 20 to 298 K) were investigated. The emission spectr a show three distinct bands, which are ascribed to ligand electronic s tates: a singlet state, S-1 (pi-pi), at 24270 cm(-1), and two triplet states, T-1 and T-2, at 15630 cm(-1) and 16610 cm(-1), respectively. Both triplet states are situated below the emitting states of the Eu3 and Tb3+ ions (D-5(o) and D-5(4), respectively), thus suppressing the luminescence from these states by back-transfer. (C) 1998 Elsevier Sc ience B.V. All rights reserved.